RESOURCES

Site Characterization Analysis Penetrometer System with Laser-Induced Fluorometry

(SCAPS-LIF)

Certification No: 96-01-021

Technology Description:

The Site Characterization and Analysis Penetrometer System (SCAPS)-Laser-Induced Fluorescence (LIF) technology is a real-time in situ subsurface field screening method for petroleum, oil and lubricants (POLs) that contain Polynuclear Aromatic Compounds (PNAs). The technology was developed through a collaborative effort of the Army, Navy, and Air Force under the auspices of the Tri-Service SCAPS Program. The system is one of a planned family of sensors collectively called the Site Characterization and Analysis Penetrometer System, or SCAPS, that will combine remote sensors with a cone penetrometer platform to provide rapid, in-situ, subsurface measurements of many different contaminants. The method uses a fiber optic-based laser-induced fluorescence (LIF) sensor system deployed with a standard 20 ton cone penetrometer. Application of the technology is limited to contaminants containing PNA compounds which fluoresce when exposed to 337 nanometer (nm) wavelength ultraviolet (UV) light.

The chemical sensing scheme utilizes a fluorescence technique in which an optical response is stimulated in PNAs present in hydrocarbon contaminants in the soil. The PNAs are excited by 337 nm UV light from a pulsed nitrogen laser, with the most effective fluorescence response coming from PNAs with three or more aromatic rings. The excitation pulse is transmitted down the probe along an optical fiber that can be up to 100 meter (m) in length, and through a sapphire window built into the side of the cone penetrometer tip. The maximum length of fiber-optic cable is limited to 100 m due to attenuation of uv-light in the optical fiber. The induced fluorescence signal from PNAs in the soil adjacent to the sapphire window is returned over a second fiber to the surface where it is dispersed with a spectrograph, and quantified with a photodiode array. The sensor output is processed by an on-board computer to provide real-time information about the intensity of the fluorescence signal as an indicator of the relative contaminant concentration. The spectral signature of the sensor output provides information about fuel product type or potential interferences.

A photodiode array detector system is used to quantify the fluorescence emission spectrum in the wavelength region of from 350 to 720 nm. As the SCAPS-LIF probe is pushed into the soil, real- time plots are generated of depth versus maximum fluorescence intensity and the wavelength at which the maximum intensity occurs. Differences in the wavelength of maximum peak intensity offer the potential to differentiate between the target analyte and background fluorescence emissions from other sources such as naturally occurring fluorophores which may be present in the soil (e.g., carbonate minerals). Standard data collection rates (a composite measurement every 2 seconds) provide a vertical spatial resolution of better than 4 cm (1½ in) for a standard push rate of 1 m/min.

The cone penetrometer and its stress sensors were not a topic of the certification evaluation, but are an industry standard addressed in ASTM Method D-3441-86. The technology is capable of making measurements from the ground surface to depths of 150 feet, when the sensor is used in conjunction with an "industry-standard" 20-ton penetrometer push vehicle. However, maximum depth of operation is governed by site-specific stratigraphy and the method is limited to sites where the cone penetrometer can be pushed to the depth of concern, through primarily unconsolidated sedimentary deposits or formations.

The sensor is intended to provide rapid, qualitative to semi-quantitative information about the distribution of subsurface petroleum contamination, and as a method to delineate the boundaries of the subsurface contaminant plume prior to installing monitoring wells or collecting soil samples. It is not intended as a complete replacement for traditional soil samples and monitoring wells; but rather to maximize the effectiveness, and minimize the number, of conventional borings. The traditional approach to site characterization, which depends on collection of discrete soil and water samples followed by laboratory analyses, is usually a slow, iterative and costly process because the samples are collected with little prior knowledge as to the extent or exact location of the contaminant plume. Significant delays occur in site characterization while samples are analyzed. Subsequent borings must be drilled with no knowledge of the results from other boring locations, or the process must stop to await results from previous sampling. Because the LIF sensor provides real-time chemical information while the system is in the field, sampling plans can be adjusted in the field to improve tracking and delineation of subsurface plumes and their boundaries.